We report on the reactivation of hepatitis B in a renal transplant patient who had been treated with rituximab for recurrent focal segmental glomerulosclerosis two and a half yr previously. He lost his anti-hepatitis B surface antigens and anti-hepatitis B core antigen antibodies and developed hepatitis B virus (HBV)-DNA positive hepatitis. Hepatitis C, which had been successfully treated by alpha interferon 10 yr before, remained quiescent. We suggest regular controls of HBV-DNA, anti-HBV antibodies and transaminases for prolonged periods in patients with status post-hepatitis B treated with rituximab. Prophylactic therapy with lamivudine and/or hepatitis B hyperimmune globulin may be considered in patients with a decrease in anti-HBV antibodies.